Actually ive just totalled up the cost of the plants in my polytunnel if i had to replace a section of them and overall the plants are worth several hundred pounds. Whilst i doubt they damaged all the plants if they did damage some I could see it coming to 120 depending on what they had in there.
It may have been more useful if they included an explanation of how they came to the figure but all in all i think assuming paying for new glass is the only cost is fairly unreasonable.
Plus are they refitting the glass themselves or does the 50 include someone to fit it.
If not then the 120 could break down to
Labour to clean up the mess
Cost of replacement plants, soil pots etc
Cost to refit the glass
Personally i think they deserve to be compensated for all 3 and if they are willing to bear the costs themselves and think something good will come out of it via charity then fair play to them, their only unreasonable action was a lack of clear communication
Of course if its an empty greenhouse and the 50 includes glass fitting then 120 is pretty steep just for the tidy up, unless you missed out that your son did the tidy up.
On balance though, as a keen gardener i think yabu to think giving them 50 for some glass and an apology just from you is sufficient